Preparation of hydroxylamines from ammonia or the corresponding amines, hydrogen and oxygen

ABSTRACT

Hydroxylamines are prepared from ammonia or the corresponding amines, hydrogen and oxygen by a process in which the starting materials are reacted under heterogeneous catalysis using an oxidation catalyst based on a titanium or vanadium silicalite having a zeolite structure and containing from 0.01 to 20% by weight of one or more platinum metals selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, the platinum metals each being present in at least two different bond energy states.

This application is a 371 of PCT/EP95/03771 filed 23 Sep. 1995.

The present invention relates to an improved process for the preparationof hydroxylamines from ammonia or the corresponding amines, hydrogen andoxygen using a certain oxidation catalyst.

Multistage industrial processes are known for the preparation ofhydroxylamine starting from ammonia. The disadvantage of these processesis that they bind the resulting hydroxylamine in the form of ammoniumsalts, which subsequently, for example when used in processes for thepreparation of caprolactam, inevitably lead to considerable amounts ofammonium sulfate salts.

A salt-free process based on ammonia and hydrogen peroxide is describedin EP-A 522 634, but the use of expensive hydrogen peroxide makes thisprocess uneconomical.

It is an object of the present invention to provide an easily preparedand efficient oxidation catalyst for the preparation of hydroxylaminesfrom ammonia or the corresponding amines and a corresponding processwhich no longer has the disadvantages of the prior art and is capable ofproducing hydroxylamines in salt-free form from ammonia or thecorresponding amines, hydrogen and oxygen.

We have found that this object is achieved by a process for thepreparation of hydroxylamines from ammonia or the corresponding amines,hydrogen and oxygen, wherein the starting materials are reacted underheterogeneous catalysis using an oxidation catalyst based on a titaniumor vanadium silicalite having a zeolite structure and containing from0.01 to 20% by weight of one or more platinum metals selected from thegroup consisting of ruthenium, rhodium, palladium, osmium, iridium andplatinum, the platinum metals each being present in at least twodifferent bond energy states.

Such oxidation catalysts are disclosed in German Patent Application P 4425 672.8.

For the purpose of the present invention, it is of decisive importancethat, before it is used, the oxidation catalyst contains the platinummetals in the stated special modification comprising the mixture ofdifferent bond energy states. The different bond energy statescorrespond formally to different oxidation states of the metals. In apreferred embodiment, two, three, four or five different bond energystates are present.

Where two different bond energy states are present, this may correspond,for example, to a mixture of species of oxidation states 0 and +1, 0 and+2, 0 and +3 or 0 and +4. The two species are usually present in a ratioof from 5:95 to 95:5, in particular from 10:90 to 90:10.

Where three different bond energy states are present, this corresponds,for example, to a mixture of species of oxidation states 0, +1 and +2 or0, +2 and +3, or 0, +2 and +4 or 0, +1 and +3 or 0, +1 and +4 or 0, +3and +4. The three species are usually present in a ratio of(0.05-20):(0.05-20):1, in particular (0.1-10):(0.1-10):1.

Mixtures of four or more different oxidation states may also be present,for example of 0, +1, +2 and +3 or 0, +1, +2 and +4, or 0, +2, +3 and +4or 0, +1, +3 and +4 or 0, +1, +2, +3 and +4. Here, the species arepresent in weight ratios which are similar to those in the case of themixtures of 2 or 3 different oxidation states.

Palladium is preferred among platinum metals. In a particularlypreferred embodiment, the palladium is present in two or three differentbond energy states.

The bond energy states at the surface of the catalyst can most easily becharacterized by X-ray photoelectron spectroscopy (XPS). For example, ina typical mixture of three palladium species, the corresponding valuesfor the energies of the Pd-3d_(5/2) state is 335.0-335.4 eV, 336-336.6eV and 337.1-337.9 eV, which formally corresponds to the oxidationstates Pd⁰, Pd¹⁺ and Pd²⁺.

In the case of the oxidation catalysts described, it is particularlyadvantageous to apply the platinum metals in such a way that nometal-metal bonds are effective and metal-zeolite bonds predominate. Inparticular, X-ray fine structure investigations (EXAFS) reveal that,with the presence of palladium, an essential feature is that virtuallyexclusively palladium-oxygen bond distances of 2.02±0.02 Å occur andpalladium-palladium distances of 2.74±0.02 Å, as in expanded palladiummetal or palladium agglomerates, and palladium-palladium distances of3.04±0.02 Å as in palladium(II) oxide are avoided.

The oxidation catalyst described is based on known titanium silicalitesor vanadium silicalites having a zeolite structure, preferably having apentasil zeolite structure, in particular the types which are classifiedas the MFI or MEL structure or MFI/MEL mixed structure by X-rayanalysis. Zeolites of this type are described, for example, in W. M.Meier and D. H. Olson, Atlas of Zeolite Structure Types, Butterworths,2nd Ed. 1987. Titanium-containing zeolites having the ZSM-48, ferrieriteor β-zeolite structure are also possible.

In the oxidation catalyst described, some or all of the titanium of thesilicalite may be replaced by vanadium. The molar ratio of titaniumand/or vanadium to the sum of silicon plus titanium and/or vanadium isas a rule from 0.01:1 to 0.1:1.

The content of the stated platinum metals in the oxidation catalystdescribed is from 0.01 to 20, preferably from 0.1 to 10, in particularfrom 0.2 to 5, % by weight, based on the total weight of the oxidationcatalyst.

Apart from being modified with the stated platinum metals, the oxidationcatalyst described may additionally be modified with one or moreelements selected from the group consisting of iron, cobalt, nickel,rhenium, silver and gold. These elements are then usually present in anamount of from 0.01 to 10, in particular from 0.05 to 5, % by weight,based on the total weight of the oxidation catalyst.

The oxidation catalyst described is advantageously prepared byimpregnating or reacting the titanium or vanadium silicalite having thezeolite structure with salt solutions, chelate complexes or carbonylcomplexes of the platinum metals, by a preparation method in which therequired distribution of the bond energy states of the platinum metalsis established after the impregnation or reaction by suitable reducingor oxidizing conditions.

Thus, the platinum metals may be applied, for example, by impregnationwith a platinum metal salt solution, in particular in oxidation states+2 to +4, from pure aqueous, pure alcoholic or aqueous alcohol mixtureat from 20° to 90° C., in particular from 30° to 55° C. The salts usedmay be, for example, the corresponding chlorides, acetates or tetraminecomplexes thereof, and palladium(II) chloride, palladium(II) acetate andthe palladium(II)-tetraminechloro complex are to be mentioned here inthe case of palladium. In this case the amount of the metal salts shouldbe chosen so that concentrations of from 0.01 to 20% by weight ofplatinum metal are achieved on the resulting oxidation catalyst.

The reaction with corresponding chelate complexes of the platinum metalsin nonpolar solvents, for example with acetylacetonates,acetonylacetonates or phosphine complexes, is also suitable here.

Application in the form of corresponding carbonyl complexes of theplatinum metals is also possible. This is advantageously carried out inthe gas phase under superatmospheric pressure or by impregnation withthese carbonyl complexes in supercritical solvents, such as CO₂.

After the resulting catalyst intermediate has been subjected to anyrequired drying and/or any required baking step, the distribution of thebond energy states is established preferably by partial reduction ofexisting high oxidation states of the platinum metals, in particular byhydrogenation in a hydrogen atmosphere. If the platinum metals arealready present in the oxidation state 0, for example on application ascarbonyl complexes, partial oxidation must be effected.

In a preferred embodiment, the oxidation catalyst described isimpregnated with salt solutions of the platinum metals in the oxidationstates +2 to +4, and the dried catalyst is then hydrogenated in ahydrogen atmosphere; in this preparation method, the hydrogenation iscarried out at from 20° to 120° C., in particular from 25° to 100° C.,preferably from 30° to 70° C.

If the temperature is chosen too high in this partial reduction byhydrogenation in a hydrogen atmosphere, the platinum metals are presentvirtually exclusively in the oxidation state 0, ie. as metals, and inthe form of relatively large agglomerates, which is detectable in themicrograph from the occurrence of metal clusters having sizes greaterthan 1.0 nm.

The abovementioned titanium or vanadium silicalites having a zeolitestructure, in particular those having the MFI pentasil zeolitestructure, are generally prepared by crystallizing a synthetic gel,consisting of water, a titanium or vanadium source and silica in asuitable manner with the addition of organic nitrogen-containingcompounds (template compounds) under hydrothermal conditions and, ifrequired, with the addition of ammonia, an alkali or fluoride asmineralizers. Examples of suitable organic nitrogen-containing compoundsare 1,6-diaminohexane or salts or the free hydroxide oftetraalkylammonium, especially of tetrapropylammonium.

In the preparation of the titanium or vanadium silicalites,contamination with relatively large amounts of alkali metal or alkalineearth metal compounds must be avoided; alkali metal contents (inparticular sodium or potassium contents) of <100 ppm are desirable inorder subsequently to obtain a sufficiently active oxidation catalyst.

The crystallization of the single-phase structure of the titanium orvanadium silicalite is effected preferably at from 140° to 190° C., inparticular from 160° to 180° C., in the course of from 2 to 7 days, aproduct having good crystallinity being obtained after only about 4days. The duration of the synthesis on the one hand and the crystallitesize on the other hand can be substantially reduced by vigorous stirringand a high pH of from 12 to 14 during the crystallization.

For example, primary crystallites of from 0.05 to 0.5 μm, in particularthose having a mean particle diameter of less than 0.2 μm, areadvantageous.

After the crystallization, the titanium or vanadium silicalite can befiltered off by a method known per se, washed and dried at from 100° to120° C.

In order to remove the amine or tetraalkylammonium compounds stillpresent in the pores, the material may furthermore be subjected to athermal treatment in air or under nitrogen. In this procedure, it isadvantageous to burn off the template under conditions which limit thetemperature increase to <550° C.

In addition to the abovementioned additions of platinum metals and otherelements, the prior art methods for shaping with the aid of a binder,ionic exchange or surface modification for example by chemical vapordeposition (CVD) or chemical derivatization, for example silylation, maybe used for modifying the oxidation catalyst described.

The presence of the catalyst functions required for an oxidationreaction may be tested by IR spectroscopy: significant bands occur at550 cm⁻¹ and at 960 cm⁻¹ and indicate the presence of the desiredcrystallinity and of the required oxidation activity.

The oxidation catalyst described can also be regenerated in a simplemanner. Deactivated catalysts can be converted back into an active formby controlled burning off and subsequent reduction with, for example,hydrogen.

If the coating is small, the catalyst can also be regenerated by asimple wash process. Depending on requirements, the wash process can becarried out at neutral, acidic or basic pH. If necessary, the catalystactivity can also be regenerated by means of a solution of hydrogenperoxide in a mineral acid.

The oxidation catalyst described is particularly suitable for thepreparation of unsubstituted hydroxylamine as well as for thepreparation of substituted hydroxylamines from the corresponding amines,hydrogen and oxygen, for example from cyclic or aliphatic amines, suchas cyclohexylamine, which can undergo partial further reaction to thecorresponding lactams under the reaction conditions.

The novel reaction can be carried out in the liquid phase, in the gasphase or in the supercritical phase. In the case of liquids, thecatalyst is preferably used as a suspension, while a fixed-bedarrangement is advantageous in the gas-phase or supercritical procedure.

If hydroxylamines are prepared in the liquid phase, the process isadvantageously carried out at from 1 to 100 bar and by a suspensionprocedure in the presence of solvents. Suitable solvents are alcohols,eg. methanol, ethanol, isopropanol or tert-butanol, or mixtures thereof,and in particular water. Mixtures of the stated alcohols with water mayalso be used. In certain cases, the use of water or water-containingsolvent systems results in a substantial increase in the selectivity ofthe desired epoxide compared with the pure alcohols as solvents.

The novel reaction is carried out as a rule at from -5° to 70° C., inparticular from 20° to 50° C. The molar ratio of hydrogen to oxygen (H₂:O₂) can usually be varied in the range from 1:10 to 1:1 and isparticularly advantageously from 1:2.5 to 1:1. The molar ratio of oxygento ammonia is as a rule from 1:1 to 1:3, preferably from 1:1.5 to 1:1.7.The carrier gas introduced may be any inert gas, nitrogen beingparticularly suitable.

The examples which follow are intended to describe the invention in moredetail without restricting it.

EXAMPLE 1

This example describes the crystallization of a titanium silicalite.

For this purpose, 455 g of tetraethyl orthosilicate were initially takenin a 2 l four-necked flask and 15 g of tetraisopropyl orthotitanate wereadded in the course of 30 minutes from a dropping funnel while stirring(250 rpm, paddle stirrer). A colorless, clear mixture formed. Finally,800 g of a 20% strength by weight aqueous tetrapropylammonium hydroxidesolution (alkali metal content <10 ppm) were added and stirring wascontinued for a further hour. The alcohol mixture (about 450 g) formedby hydrolysis was distilled off at from 90° to 100° C. The mixture wasmade up with 1.5 l of demineralized water, and the now slightly opaquesol was transferred to a stirred 2.5 l autoclave. The closed autoclave(anchor stirrer, 200 rpm) was brought to a reaction temperature of 175°C. at a heating rate of 3° C./min. The reaction was complete after 92hours. The cooled reaction mixture (white suspension) was centrifugedand the resulting solid was washed neutral several times with water. Thesolid obtained was dried at 110° C. in the course of 24 hours (weightobtained 149 g). Finally, the template still present in the zeolite wasburnt off under air at 500° C. in the course of 5 hours (loss oncalcination: 14% by weight).

The pure white product had a titanium content of 1.5% by weight and aresidual alkali metal content (potassium) of 0.01% by weight, accordingto wet chemical analysis. The yield (based on SiO₂ used) was 97%. Thecrystallite size was about 0.1-0.15 μm and the product showed bands at960 cm⁻¹ and 550 cm⁻¹, which are typical for the IR spectrum.

EXAMPLE 2

For impregnation with palladium, a flesh-colored solution was firstprepared using 0.515 g of palladium(II) chloride and 120 g of ammoniasolution (25% by weight in water) while stirring at room temperature. 60g of the freshly prepared titanium silicalite from Example 1 weresuspended in 130 g of demineralized water in a round-bottomed flask. Thetotal amount of the prepared palladium-tetraminechloro complex solutionwas added to this, and the mixture was stirred for one hour in a rotaryevaporator at room temperature under atmospheric pressure. Finally, thesuspension was evaporated down under reduced pressure (5 mbar) at90°-100° C. The white product was used directly for the reduction.

In a laboratory rotary tubular furnace (quartz glass, diameter 5 cm,length of heating zone 20 cm), 20 g of the Pd-impregnated product werereduced in the course of 90 minutes at 50° C. with a gas mixturecomprising 20 l/h of nitrogen and 1 l/h of hydrogen at a rotationalspeed of the furnace of 50 rpm.

The finished product had a pale color and had no metallic palladiumclusters larger than 1.0 nm according to analysis under the transmissionelectron microscope (TEM). The palladium content was determined at 0.49%by weight by a wet chemical method. The three abovementioned bond energystates of the Pd-3d_(5/) 2 photoelectron (formally corresponding to theoxidation states +2, +1 and 0) were found by means of XPS.

EXAFS measurements on this sample gave a signal for Pd-O or Pd-N bonddistances of 2.02±0.02 Å. Pd-Pd bond distances of 2.74±0.02 Å or3.04±0.02 Å were not observed.

EXAMPLE 3

This example illustrates the one-stage preparation of hydroxylamine fromammonia, hydrogen and oxygen over the catalyst prepared according toExamples 1 and 2, in aqueous solution.

2 g of catalyst (Example 2) were initially taken with 150 ml ofdistilled water and 2.4 g of ammonia solution (25% by weight in water)in a pressure-resistant steel reactor (volume 0.375 l). 20 bar oxygenand 20 bar hydrogen were then introduced into the closed reactor.

The stirred suspension was reacted at 30° C. for 1 hour, then cooled andfreed from excess hydrogen/oxygen mixture by introducing in nitrogen andletting down the pressure, these steps being carried out three times.

Hydroxylamine formed was determined by titration and derivatization. Theyield was 11%, based on the ammonia used.

EXAMPLE 4

This example illustrates the requirement for the presence of noblemetals on the novel catalyst.

The experiment in Example 3 was repeated with 2 g of catalyst fromExample 1.

No hydroxylamine was found in the discharged mixture.

EXAMPLE 5

This example illustrates the requirement for the use of hydrogen inorder to be able to react ammonia with oxygen to give hydroxylamine.

The experiment in Example 3 was repeated with the catalyst from Example2, but the procedure was carried out without the addition of hydrogen.

No hydroxylamine was observed in the discharged mixture.

We claim:
 1. A process for preparing a hydroxylamine,comprising:reacting ammonia or an amine with hydrogen and oxygen underheterogenous catalysis in the presence of an oxidation catalystcomprising titanium silicalite or vanadium silicalite having a zeolitestructure and 0.01 to 20% by weight of one or more platinum metalsselected from the group consisting of ruthenium, rhodium, palladium,osmium, iridium and platinum, wherein each pialinum metal is present inat least two different bond energy states.
 2. A process as claimed inclaim 1, wherein the oxidation catalyst comprises from 0.01 to 20% byweight of palladium, wherein the palladium is present in two or threedifferent bond energy states.
 3. A process as claimed in claim 1,wherein the oxidation catalyst further comprises one or more elementsselected from the group consisting of iron, cobalt, nickel, rhenium,silver and gold.
 4. A process as claimed in claim 1, wherein theoxidation catalyst has a molar ratio of titanium or vanadium to the sumof silicon plus titanium or vanadium of from 0.01:1 to 0.1:1.
 5. Aprocess as claimed in claim 1, wherein the oxidation catalyst isprepared by a process comprising impregnating or reacting a titaniumsilicalite or vanadium silicalite having a zeolite structure with saltsolutions, chelate complexes or carbonyl complexes of the platinummetals followed by establishing the required distribution of the bondenergy states of the platinum metals by suitable reducing or oxidizingconditions.
 6. A process as claimed in claim 5, wherein the oxidationcatalyst used is prepared by a process comprising impregnating thetitanium silicalite or vanadium silicalite with salt solutions of theplatinum metals in the oxidation states +2 to +4 and subsequentlyhydrogenating the dried catalyst in a hydrogen atmosphere at from 20° to120° C.
 7. A process as claimed in claim 1, wherein the reaction iscarried out in the presence of water.